Basic techniques of silver halide emulsions to achieve enhancement of sensitivity and image quality of silver halide photographic light sensitive materials include enhancements of monodispersibility of a silver halide grain emulsion. Optimum conditions for chemical sensitization of a silver halide emulsion depend partly on the size of silver halide grains, so that it is rather difficult to achieve optimal chemical sensitization of a polydisperse silver halide emulsion (having a broad grain size distribution), which often leads to increased fogging or insufficient chemical sensitization. On the other hand, it is easy to provide optimal chemical sensitization to a monodisperse silver halide grain emulsion, leading to a silver halide emulsion with enhanced sensitivity and low fogging. A monodisperse emulsion results in a photographic material with high contrast.
Preparation of silver bromide or silver iodobromide grains having two parallel twin planes employs the Ostwald ripening process, wherein only nucleus grains produced at the initial stage of nucleation, which have two parallel twin planes nucleus grain and growth activity in its side-faces, selectively retained by receiving solutes which resulted from dissolution of regular crystal grains. The nucleus grains are allowed to grow by the double jet addition of silver nitrate and halide solutions at a relatively high pBr, with maintaining or narrowing the size distribution of the nucleus grains. However, excessive Ostwald ripening increases the number of nucleus grains and at the same time tends to cause deterioration in homogeneity of nucleus grain size distribution. Therefore, to prepare tabular grains with highly narrow size distribution, it is desirable to narrow the size distribution at the stage of forming the tabular nucleus grains.
There have been disclosed techniques regarding monodisperse tabular silver halide grains, for example, JP-A 1-213637 (herein, the term, JP-A means unexamined and published Japanese Patent Application) discloses a technique of improving sensitivity and graininess by using monodisperse silver halide grains having two parallel twin planes. JP-A 5-173268 and 6-202258 disclose a method for preparing tabular silver halide grains with narrow grain size distribution. These techniques are directed to monodisperse tabular silver halide grains with a small variation in the area-equivalent grain diameter.
Tabular grain size is generally defined in terms of two parameters. The first parameter is an area-converted grain diameter and the other one is a grain thickness. Thus, the size distribution of tabular grains cannot be achieved, even if only the area-converted grain diameter distribution of tabular grains is narrowed. Broad size distribution of tabular grains produces problems such as fluctuation in sensitivity deterioration in graininess due to difference in developability.
JP-A 6-258744 teaches a technique for improving pressure resistance and latent image stability as well as enhanced sensitivity and contrast by using monodisperse tabular silver halide grains with an aspect ratio of 2 or more and having internal portions different in halide composition, wherein the monodisperse silver halide grains means those having a small fluctuation in the volume-converted grain diameter. However, this technique does not include anything with respect to the manufacturing cost of the tabular silver halide grain emulsion.
JP-A 5-210188 discloses a technique of forming core grains containing 10 to 45 mol % iodide while maintaining 0.1 to 3.0 .mu.m of the intergrain distance. However, this technique suggests neither intention of controlling the mean intergrain distance in the process of grain growth nor means therefor.
Development of silver halide emulsions including a tabular silver halide grain emulsion is associated with problems regarding manufacturing costs as well as photographic performance. An effective method for controlling the manufacturing cost is to increase the emulsion-manufacturing amount per run. In other words, this means to increase the yield of silver halide at the time when completing grain growth in the reaction vessel used for manufacturing silver halide emulsions. Thus, it is to enhance the silver halide concentration of the emulsion at the completion of grain growth and a direct method is to concentrate a silver halide emulsion or reduce the volume of the emulsion. JP-B 59-43727 (herein, the term JP-B means examined and published Japanese Patent) and JP-A 3-140946 disclose techniques of using ultrafiltration to reduce the volume of reaction product (i.e. a silver halide emulsion) in the process of preparing emulsions. However, these disclosures do not include any suggestion with respect to tabular grains nor monodisperse tabular grain emulsions. Further, these techniques were not intended to control the mean intergrain distance of silver halide grains in the preparation of silver halide emulsions.
JP-A 6-67326 discloses a method in which ultrafiltration is applied to the preparation of tabular silver halide grain emulsions to reduce the reaction product volume to obtain tabular grains with medium aspect ratios (i.e., 2 to 8), together with an enhanced yield. According to the disclosure, employing the aspect ratio of silver halide grains systemtically decreasing with the concentration, tabular grains with a medium aspect ratio were obtained by concentrating a high aspect ratio tabular grain emulsion through the ultrafiltration in the course of the preparation thereof. However, the higher the aspect ratio is, it is generally difficult to enhance homogeneity of the tabular grains, so that it cannot be expected to achieve a sufficient improvement in the grain size distribution by the method of reducing the aspect ratio through concentration of a tabular grain emulsion with high aspect ratio. In fact, silver halide emulsions described in the disclosure, including comparative and inventive emulsions, exhibited 0.3 or more of a coefficient of variation of volume-converted diameter, therefore, it is insufficient to solve problems associated with photograpic performance of tabular grains. Moreover, the disclosure does not teach about any apparatus for preparing the silver halide emulsions.
There have been proposals of enhancing the sensitivity of silver halide emulsions. Specifically, there is known a technique of introducing iodide into the silver halide grain surface to promote adsorption of a spectral sensitizing dye. Disclosed as a method for introducing iodide into the grain surface are addition of potassium iodide to an emulsion to cause halide conversion, simultaneous addition of silver nitrate and potassium iodide solutions, as described in JP-A 4-107442 and addition of a fine silver iodide grain emulsion. JP-A 2-68538 and 5-323487 disclose a technique of forming silver halide grains by the use of an iodide ion releasing agent. However, the iodide ion releasing agent, which releases an iodide ion upon reaction with a base or a nucleophilic agent, produced problems such that a by-product or an unreacted iodide ion releasing agent remain in the reaction mixture, leading to deterioration in homogeneity of the iodide distribution among the grains.